Current themes in neuroimaging studies of reading

The vertical superior longitudinal fascicle and the vertical occipital fascicle

Association fibers of the human brain have long been considered to exclusively follow an anterior-posterior direction. Using magnetic resonance imaging techniques that allow in-vivo fiber dissection, vertically oriented association fibers have been rediscovered or newly described. Aside from the frontal aslant tract (FAT) in the frontal lobe, the vertical occipital fascicle (VOF) and the vertical portion of the superior longitudinal fascicle system (vSLF) have been studied in recent years. The aim of this review was to give an overview on the current knowledge regarding these two fiber tracts. A review of the available literature in the Medline database was conducted to gather all available publications dealing with either the VOF or the vSLF. One thousand two hundred seventy-three articles were obtained from the literature search of which a total of 71 articles met the final inclusion criteria of this review. We describe the history of the discovery of the respective fiber tract, its anatomical course and its boundaries integrating blunt fiber dissection studies and functional MRI/tractography studies. We discuss the functional properties of the respective fiber tract and its relevance in neurosurgery. The VOF is a fiber tract that has been discovered in the late XIX century and long been forgotten before being rediscovered in the 1970's. It lies lateral to the fibers of the sagittal stratum and mainly connects the superior and inferior occipital lobe. It plays a major role in reading and visual word and language comprehension and is said to be the main link between dorsal and ventral visual streams. The vSLF has many synonyms and is part of the superior longitudinal fascicle system. Recent studies were able to provide more insight into this set of fiber tracts showing distinct connections running from the superior and inferior parietal lobule to the posterior part of the temporal lobe. Its functional role is still not completely cleared. It is said to play a role in visual and auditory semantic language comprehension. It lies directly lateral to the arcuate fascicle. The VOF and the vSLF are vertically oriented fiber tracts connecting the temporo-parieto-occipital region and play a major role in the communication of dorsal and ventral visual streams (VOF), reading (VOF, vSLF) and visual and auditory semantic language comprehension (vSLF). They can consistently be identified using ex vivo blunt dissection techniques and in-vivo fiber tractography. Because of their localization and orientation these two fiber tracts can be combined to a fiber bundle system called posterior transverse system (PTS).

Validation of The Reading House and Association With Cortical Thickness

BACKGROUND AND OBJECTIVES

The American Academy of Pediatrics recommends literacy and school readiness promotion during well visits. The Reading House (TRH) is a children's book-based screener of emergent literacy skills in preschool-aged children. Vocabulary, rhyming, and rapid naming are core emergent skills, and reading abilities are associated with thicker cortex in the left hemisphere. Our objective was to expand validity of TRH relative to these skills and explore association with cortical thickness.

METHODS

Healthy preschool-aged children completed MRI including a T1-weighted anatomic scan. Before MRI, TRH and assessments of rapid naming (Comprehensive Test of Phonological Processing, Second Edition), rhyming (Pre-Reading Inventory of Phonological Awareness), vocabulary (Expressive Vocabulary Test, Second Edition), and emergent literacy (Get Ready to Read!) were administered. Analyses included Spearman-ρ correlations (r _ρ) accounting for age, sex, and socioeconomic status (SES). MRI analyses involved whole-brain measures of cortical thickness relative to TRH scores, accounting for covariates.

RESULTS

Seventy children completed assessments (36-63 months old; 36 female) and 52 completed MRI (37-63 months; 29 female). TRH scores were positively correlated with Comprehensive Test of Phonological Processing, Second Edition (r _ρ = 0.61), Expressive Vocabulary Test, Second Edition (r _ρ = 0.54), Get Ready to Read! (r _ρ = 0.87), and Pre-Reading Inventory of Phonological Awareness scores (r _ρ = 0.64; all P < .001). These correlations remained statistically significant across age, sex, and SES groups. TRH scores were correlated with greater thickness in left-sided language and visual cortex (P-family-wise error <.05), which were similar for higher SES yet more bilateral and frontal for low SES, reflecting a less mature pattern (P-family-wise error <.10).

CONCLUSIONS

These findings expand validation evidence for TRH as a screening tool for preschool-aged children, including associations with emergent skills and cortical thickness, and suggest important differences related to SES.

A Connectomic Atlas of the Human Cerebrum-Chapter 16: Tractographic Description of the Vertical Occipital Fasciculus

In this supplement, we show a comprehensive anatomic atlas of the human cerebrum demonstrating all 180 distinct regions comprising the cerebral cortex. The location, functional connectivity, and structural connectivity of these regions are outlined, and where possible a discussion is included of the functional significance of these areas. In this chapter, we specifically address regions integrating to form the vertical occipital fasciculus.

Functional MRI in medulloblastoma survivors supports prophylactic reading intervention during tumor treatment

Development of reading skills is vulnerable to disruption in children treated for brain tumors. Interventions, remedial and prophylactic, are needed to mitigate reading and other learning difficulties faced by survivors. A functional magnetic resonance imaging (fMRI) study was conducted to investigate long-term effects of a prophylactic reading intervention administered during radiation therapy in children treated for medulloblastoma. The fMRI study included 19 reading-intervention (age 11.7 ± 0.6 years) and 21 standard-of-care (age 12.1 ± 0.6 years) medulloblastoma survivors, and 21 typically developing children (age 12.3 ± 0.6 years). The survivors were 2.5 [1.2, 5.4] years after completion of tumor therapies and reading-intervention survivors were 2.9 [1.6, 5.9] years after intervention. Five fMRI tasks (Rapid Automatized Naming, Continuous Performance Test using faces and letters, orthographic and phonological processing of letter pairs, implicit word reading, and story reading) were used to probe reading-related neural activation. Woodcock-Johnson Reading Fluency, Word Attack, and Sound Awareness subtests were used to evaluate reading abilities. At the time of fMRI, Sound Awareness scores were significantly higher in the reading-intervention group than in the standard-of-care group (p = 0.046). Brain activation during the fMRI tasks was detected in left inferior frontal, temporal, ventral occipitotemporal, and subcortical regions, and differed among the groups (p < 0.05, FWE). The pattern of group activation differences, across brain areas and tasks, was a normative trend in the reading-intervention group. Standardized reading scores and patterns of brain activation provide evidence of long-term effects of prophylactic reading intervention in children treated for medulloblastoma.

Forced-Attention Dichotic Listening With University Students With Dyslexia: Search for a Core Deficit

Rapidly changing environments in day-to-day activities, enriched with stimuli competing for attention, require a cognitive control mechanism to select relevant stimuli, ignore irrelevant stimuli, and shift attention between alternative features of the environment. Such attentional orchestration is essential to the acquisition of reading skills. In the present forced attention dichotic listening study, adults with moderate and severe dyslexia and nondisabled adults were tested on their ability to switch attention between ears for immediate recall. Blocks of pairs of consonant-vowel syllables were counterbalanced into left-ear first or right-ear first ordered conditions. Significant order effects showed that only those with severe dyslexia were poorer in switching attention to the left ear, whereas both groups with dyslexia were poorer switching attention to the right ear. Shifting left appears to be a normative function of reading level, whereas inferior ability to disengage attending to the left ear to report from the right ear qualifies as a dysfunctional facet of dyslexia with etiological significance. No support was found for the traditional proposition that dyslexia may be associated with atypical left hemisphere lateralization. Combining these results with previous dichotic and neuroimaging research implicates a dysfunctional frontostriatal cognitive control network in dyslexia. With due caution, the results suggest that a neurobiological feature of dyslexia may be a lack of control in downwardly modulating excessive left inferior frontal cortex activations. The results are consistent with impoverished connectedness between left anterior and posterior language areas and, pending future confirmation of these findings, suggest the need for a reconceptualization of remedial programming.

New insights into the neural network mediating reading processes provided by cortico-subcortical electrical mapping

OBJECTIVES

To ascertain the neural network mediating reading using intraoperative electrostimulation.

EXPERIMENTAL DESIGN

A cortical and axonal intraoperative electrical mapping of reading processes was achieved in seven patients who underwent awake surgery for a left occipitotemporal glioma. We performed resection cavity overlapping and superimposition with a diffusion tensor imaging-based white matter atlas. We assessed the relationship between the location of resection cavities and the occurrence of reading impairments of regular, irregular, and pseudowords.

PRINCIPAL OBSERVATIONS

Intraoperative stimulation of the left posterior inferior temporal cortex (ITCp) elicited reading disturbances. Subcortical stimulation at the anterior portion of the visual word form area (VWFA) induced addressed phonology (irregular words reading) disturbances. Subcortical stimulation of the connection between VWFA and the posterior segment of the arcuate fascicle (AFp) induced both addressed and assembled phonology (irregular and pseudowords reading) disturbances. Postoperative assessment showed that resection of the posterior portion of the inferior longitudinal fascicle (ILFp), connecting the visual cortex to VWFA, induced long-term and global reading impairment. Resection of the terminations of left AFp in the ITCp-induced irregular and pseudowords reading disturbances with no impairment of regular words reading. Resection of the anterior portion of ILF did not induce reading impairment.

CONCLUSIONS

Our data support an inner posterior-to-anterior hierarchical coding of letter strings in the VWFA and a crucial role of the left ILFp to provide visual inputs to the VWFA. Furthermore, we suggest that the AFp is involved in an interactive feedback system between visual and nonvisual information, recruited when reading irregular and pseudowords.

The level of audiovisual print-speech integration deficits in dyslexia

The classical phonological deficit account of dyslexia is increasingly linked to impairments in grapho-phonological conversion, and to dysfunctions in superior temporal regions associated with audiovisual integration. The present study investigates mechanisms of audiovisual integration in typical and impaired readers at the critical developmental stage of adolescence. Congruent and incongruent audiovisual as well as unimodal (visual only and auditory only) material was presented. Audiovisual presentations were single letters and three-letter (consonant-vowel-consonant) stimuli accompanied by matching or mismatching speech sounds. Three-letter stimuli exhibited fast phonetic transitions as in real-life language processing and reading. Congruency effects, i.e. different brain responses to congruent and incongruent stimuli were taken as an indicator of audiovisual integration at a phonetic level (grapho-phonological conversion). Comparisons of unimodal and audiovisual stimuli revealed basic, more sensory aspects of audiovisual integration. By means of these two criteria of audiovisual integration, the generalizability of audiovisual deficits in dyslexia was tested. Moreover, it was expected that the more naturalistic three-letter stimuli are superior to single letters in revealing group differences. Electrophysiological and hemodynamic (EEG and fMRI) data were acquired simultaneously in a simple target detection task. Applying the same statistical models to event-related EEG potentials and fMRI responses allowed comparing the effects detected by the two techniques at a descriptive level. Group differences in congruency effects (congruent against incongruent) were observed in regions involved in grapho-phonological processing, including the left inferior frontal and angular gyri and the inferotemporal cortex. Importantly, such differences also emerged in superior temporal key regions. Three-letter stimuli revealed stronger group differences than single letters. No significant differences in basic measures of audiovisual integration emerged. Convergence of hemodynamic and electrophysiological signals appeared to be limited and mainly occurred for highly significant and large effects in visual cortices. The findings suggest efficient superior temporal tuning to audiovisual congruency in controls. In impaired readers, however, grapho-phonological conversion is effortful and inefficient, although basic audiovisual mechanisms seem intact. This unprecedented demonstration of audiovisual deficits in adolescent dyslexics provides critical evidence that the phonological deficit might be explained by impaired audiovisual integration at a phonetic level, especially for naturalistic and word-like stimulation.

Connectionist neuropsychology: uncovering ultimate causes of acquired dyslexia

Acquired dyslexia offers a unique window on to the nature of the cognitive and neural architecture supporting skilled reading. This paper provides an integrative overview of recent empirical and computational work on acquired dyslexia within the context of the primary systems framework as implemented in connectionist neuropsychological models. This view proposes that damage to general visual, phonological or semantic processing abilities are the root causes of different forms of acquired dyslexia. Recent case-series behavioural evidence concerning pure alexia, phonological dyslexia and surface dyslexia that supports this perspective is presented. Lesion simulations of these findings within connectionist models of reading demonstrate the viability of this approach. The commitment of such models to learnt representations allows them to capture key aspects of performance in each type of acquired dyslexia, particularly the associated non-reading deficits, the role of relearning and the influence of individual differences in the premorbid state of the reading system. Identification of these factors not only advances our understanding of acquired dyslexia and the mechanisms of normal reading but they are also relevant to the complex interactions underpinning developmental reading disorders.